Re: feelings of guilt
Of course it's normal. You've had your cat for most of your life and I understand it's very hard. I've gone through 3 dogs, 1 skinny pig, 3 budgies, 2 turtles, and 1 chinchilla. It's never easy. As for the guilt, if we were omnipotent we'd know what to do in every instance and be able to everything perfectly, but we're only human. We do what we can with the info that we have at that moment. You did what you could. Just know that you gave your cat the best life that it could have with the knowledge and experience you possess at this time.
When my chinchilla passed, I was beyond upset because I felt exactly the same as you do. Perhaps if I bought him in sooner he would be saved. Perhaps not. I do know that when he was alive, I did treated him well and did I what I could for him at that time to lead a happy life.
It may or may not work for you, but for me...I adopted another chinchilla at the shelter after about 2 months. He's helped me deal with my first chinchilla's passing. At first, I wasn't ready to deal with the prospect of another pet...another possibility of them passing before you. It would be devastating to me. But, I realized with the knowledge that I possess, it would be selfish to not save and help another chinchilla. They're exotic pets and most people have no idea how to handle them. I felt I had to use my expertise to help. Even one would be better than none.
So yes, it's perfectly normal to go through the feelings of guilt, sadness etc. Time won't completely get rid of it, but you'll feel better after a while.